1. Field of the Invention
The present invention relates to a system and method of transmitting an optical signal of a single wavelength or a wavelength-division multiplexed optical signal, and in particular, to a technique which is effectively applied to an optical transmission system for modulating an optical signal by using a partial response encoded signal. The present invention also relates to an optical transmission system for suppressing the degradation of the transmitting quality due to chromatic dispersion of a transmission medium such as an optical fiber, or to interaction between the chromatic dispersion and the nonlinear optical effects in the transmission medium. The present invention also relates to an optical transmitter and optical receiver which constitute the optical transmission system.
2. Description of the Related Art
In conventional optical fiber transmission systems, various kinds of encoded signals for modulation have been proposed for improving tolerance with respect to waveform distortion due to chromatic dispersion of a relevant optical fiber, and for reducing the wavelength distortion due to the nonlinear optical effects occurring in a relevant optical fiber transmission path.
As a disclosed technique for improving chromatic dispersion tolerance, Reference 1 (K. Yonenaga et al., “Dispersion-Tolerant Optical Transmission System Using Duobinary Transmitter and Binary Receiver”, Journal of Lightwave Technology, LT-15, (8), pp. 1530-1537, 1997) discloses an optical duobinary modulating means which has a push-pull type Mach-Zehnder optical intensity modulator (called “MZ optical intensity modulator”) and which uses a duobinary encoded signal as a modulated signal, where the duobinary encoded signal is a three-level partial response encoded signal.
The transmitter of a conventional optical transmission system (see FIG. 37A) includes a duobinary encoding section (electric partial response encoding section) 6 for receiving a binary NRZ (non-return-to-zero) encoded signal supplied from a binary NRZ digital signal source which is in synchronism with a system clock source 2, and for outputting an electric duobinary encoded signal.
A binary NRZ encoded signal P3 (see FIG. 38A) generated by the binary NRZ digital signal source 5 is logically inverted in a logical inversion circuit 62 in the electric partial response encoding section 6 into an inverted NRZ encoded signal P4 (see FIG. 38B). This logically inverted encoded signal is converted by a pre-coder 61 having an exclusive OR (EX-OR) circuit 63 and a 1-bit delay circuit 64 (i.e., a 1-time slot delay for data having a transmission speed (or rate) B (refer to FIG. 39C)). After that, a binary NRZ pre-coder output signal P5 (see FIG. 38C) is differentially output by a differential converter 65.
The above binary NRZ pre-coder output signal P5 is amplified in an amplifying circuit 66, and then input into a low-pass filter (LPF) 67 whose 3 dB bandwidth is B/4, thereby obtaining a three-level complementary duobinary encoded signal P6 (see FIG. 38E). An equivalent circuit of LPF 67 is a pre-coder consisting of a 1-bit delay circuit 67A and an adder 67B (see FIG. 37B), so that it is obvious that signal P6 is equal to the sum of a binary NRZ pre-coder output signal P5a and a 1-bit delayed binary NRZ pre-coder output signal P5b (see FIGS. 38C and 38D).
In an optical modulating section 7, a push-pull type MZ optical intensity modulator 71 modulates a single mode optical signal P1 (see FIG. 39A), output from a continuous wave (CW) laser source 42, according to the three-level complementary duobinary encoded signal P6, and is converted into an optical duobinary encoded signal P7 (see FIG. 39B).
The above Reference 1 shows a structure, as shown in FIG. 37A, by which the chromatic dispersion tolerance can be twice as much as that of generally known NRZ encoded signals.
Another Reference 2 (A. Matsuura et al., “High-Speed Transmission System Based on Optical Modified Duobinary encoded signals”, Electronics Letters, Vol. 35, No. 9, pp. 1-2, 1999) discloses an optical partial response modulating means suitable for a system using a modified duobinary encoded signal as a modulated signal, which is also a three-level partial response encoded signal. In the relevant system, the chromatic dispersion tolerance is also increased to twice as much as that related to general NRZ encoded signals.
In order to reduce an undesirable effect of waveform distortion due to the nonlinear optical effects, a method using an RZ (return-to-zero) encoded signal having a fixed pulse width is effective. Reference 3 (K. Sato et al., “Frequency Range Extension of Actively Mode-Locked Lasers Integrated with Electroabsorption Modulators Using Chirped Grating”, Journal of Selected Topics in Quantum Electronics, Vol. 3, No. 2, pp. 250-255, 1997) discloses a relevant technique using a mode-locked laser, Reference 4 (M. Suzuki et al., “New Application of Sinusoidal Driven InGaAsP Electroabsorption Modulator to In-Line Optical Gate with ASE Noise Reduction Effect, Journal of Lightwave Technology, Vol. 10, pp. 1912-1928, 1992) discloses a relevant technique using an absorption-type semiconductor modulator, and Reference 5 (K. Iwatsuki et al., “Generation of Transform Limited Gain-Switched DFB-LD Pulses<6 ps with Linear Fiber Compression and Spectral Window”, Electronics Letters, Vol. 27, pp. 1981-1982, 1991) discloses a relevant technique using gain switching of a semiconductor laser.
None of the above References 3 to 5 discloses a data conversion encoded signal of an RZ pulse sequence.
As an example of a dual-mode beat pulse sequence generating means, Reference 6 (D. Wake et al., “Optical Generation of Millimeter-Wave Signals for Fiber-Radio Systems Using a Dual-Mode DFB Semiconductor Laser”, IEEE Transactions on Microwave Theory and Techniques, Vol. 43, pp. 2270-2276, 1995) discloses a technique for generating a dual-mode beat pulse signal by synchronizing two single-longitudinal-mode laser sources, Reference 7 (K. Sato et al., “Dual-Mode Operation of 60-GHz Mode-Locked Semiconductor Lasers”, Proceedings of the 1999 IEICE (Institute of Electronics, Information and Communication Engineers) Electronics Society Conference, C-4-8, p. 235, 1999) discloses a technique for generating a dual-mode beat pulse signal by using a mode-locked semiconductor laser, and Reference 8 (Y. Miyamoto et al., “320 Gbits/s (8×40 Gbits/s) WDM Transmission over 367 km with 120 km Repeater Spacing Using Carrier-Suppressed Return-to-Zero Format”, Electronics Letters, Vol. 35, No. 23, pp. 2041-2042, 1999) discloses a technique for generating a dual-mode beat pulse signal by using an LN (LiNbO3) MZ modulator.
Neither of the above References 6 and 7 discloses usage of a baseband signal as a modulated signal, and the above Reference 8 discloses usage of an NRZ encoded signal in synchronism with beat frequency B.
However, in the above-described conventional technique, when a binary optical partial response modulated signal such as an optical duobinary encoded signal or optical modified duobinary encoded signal is used, the same codes may successively appear (such as a sequence having a pattern of “1, 1, . . . 1”) in an optical modulated signal which is dependent on the pattern of an input binary NRZ encoded signal. In this case, the pulse width of the optical modulated signal is not constant. Therefore, if the optical input power increases, marked waveform distortion appears due to interaction between the self phase-modulation effect and the chromatic dispersion, and thus the tolerance characteristics of the chromatic dispersion are degraded.
On the other hand, in order to equalize or balance the chromatic dispersion in an optical transmission path, it is easy to provide a dispersive medium, which has dispersive characteristics opposite to those of the transmission path, in a receiver or an inline optical amplifying repeater, and to compensate the dispersion so as to have a total dispersion (value) D of 0. This condition is also preferable for the measurement of dispersion in the optical fiber transmission path.
However, in the conventional binary optical partial response modulated signal, the optimum total dispersion D is generally shifted to an anomalous dispersion (D>0) region. Therefore, if dispersion compensation is performed under the simple condition of “D=0”, considerable intersymbol interference due to the chromatic dispersion may occur between the encoded signals in the receiver because the optimum value of the dispersion compensation is shifted from that point. Accordingly, the receiving sensitivity is degraded.
Additionally, in the conventional binary optical partial response modulated signal, the initial intersymbol interference between the encoded bits in the modulated waveform is larger than that of generally known NRZ encoded signals; therefore, the receiving sensitivity tends to be degraded in a binary receiving circuit which is also applied to the NRZ encoded signals.
If a conventional optical pulse sequence having a constant pulse width is modulated using a partial response encoded signal so as to prevent waveform degradation due to the nonlinear optical effects or to prevent the intersymbol interference between encoded signals of initial modulated waveforms, then the chromatic dispersion tolerance with respect to the partial response encoded signal is considerably degraded.
Furthermore, in the conventional RZ modulation method (refer to the above References 4 to 6), the phases of each optical pulse are the same as shown in FIG. 40A (the temporal waveform of an RZ encoded signal is shown in FIG. 41A). Therefore, in the Fourier transform of a conventional optical pulse sequence signal, modes of clock components are generated at points away from the carrier (f0) component by B (transmission speed), as shown in FIG. 40B. When each of the three modes is modulated by a general NRZ encoded signal having a bandwidth of 2B, the total bandwidth is 4B as shown in FIG. 41B.
That is, the band occupied by the optically modulated spectrum of a pulse sequence is wide such as 3B to 4B or more (B is the transmission speed). Therefore, the effect of the chromatic dispersion or a dispersion slope cannot be ignored, so that the transmittable distance may be limited if the transmission speed is increased.
In addition, in a wavelength-division multiplexed system, if the band occupied by the optically modulated spectrum is wide, the number of wavelength channels which can be multiplexed in a specific optical gain band of an optical amplifier, used in the wavelength-division multiplexed system, is decreased and the signal spectrum efficiency is degraded. Therefore, the total transmission capacity of the wavelength-division multiplexed system is reduced.
In addition, in the technique of generating the dual-mode beat pulse disclosed in the above Reference 7, it is difficult to synchronize the optical frequencies of two longitudinal modes, so that the stability is inferior. The above Reference 8 also discloses a dual-mode beat pulse signal; however, the disclosed modulated data signal is a conventional NRZ encoded signal, and each line spectrum in the optical spectrum is preset at intervals corresponding to the transmission speed B. As a result, if the input power into an optical fiber exceeds the threshold of the stimulated Brillouin scattering (a few mW at a wavelength of 1.5 μm in a single-mode silica fiber), the signal is backward-scattered to the input side by the stimulated Brillouin scattering, so that the input power from a transmitter into the optical fiber (i.e., optical fiber transmission path) is considerably limited. In order to solve this problem, an additional circuit for enlarging the line width of the optical carrier signal, or the like, is necessary in conventional systems.
In other words, the degradation of the transmission quality in such an optical transmission system is caused by an effect of the group velocity dispersion of each optical fiber in the bandwidth of an optical signal. According to such an effect, the waveform of the optical pulse is deformed and interference between adjacent time slots may occur.
In order to suppress such degradation due to the group velocity dispersion, an optical duobinary transmission method using an optical transmission system as shown in FIG. 42 has been proposed (refer to Japanese Unexamined Patent Application, First Publication No. Hei 9-236781).
In FIG. 42, a binary data signal (i.e., binary signal) is input into an encoded signal conversion circuit 171 and is converted into a three-level duobinary encoded signal. This duobinary encoded signal is divided into two portions, and one of them is logically-inverted in an inversion circuit 172; then the band thereof is limited by amplitude control circuits 173-1 and 173-2, so that the signal is used for push-pull-driving a dual-electrode MZ optical intensity modulator 174 whose transmittance is biased to the minimum value.
The optical intensity of a continuous-wave signal output from a continuous-wave light source 175 is modulated according to the above duobinary encoded signals having opposite phases, and this intensity-modulated signal, that is, the optical duobinary encoded signal, is output into an optical transmission medium 103.
The optical duobinary encoded signal transmitted through the optical transmission medium 103 is directly detected by an optical detection circuit 181, and the detected signal is identified by a decision circuit 182. The logic of the signal output from the decision circuit 182 is inverted in an inversion circuit 183, thereby reproducing a binary data signal.
In the above optical duobinary transmission system, a high chromatic dispersion tolerance of the optical fiber can be obtained (refer to K. Yonenaga et al., “Optical Duobinary Transmission System with No Receiver Sensitivity Degradation”, Electronics Letters, Vol. 31, No. 4, pp. 302-304, 1995).
However, if the intensity of light incident on an optical fiber transmission path (i.e., fiber input (or launched) power) is increased in the relevant conventional structure, the dispersion tolerance is degraded. FIG. 43 shows results of a computer simulation of the dispersion tolerance of each of the optical duobinary transmission methods, and generally known methods using NRZ and RZ encoded signals. The common condition is to transmit the signal through 2 spans of 100 km of a single mode fiber having a local dispersion of +2 ps/nm/km via an optical amplifier, and the graph shows contour lines when each eye opening is degraded by 1 dB. Here, the graph also shows the dispersion tolerance of the present invention explained below.
In FIG. 43, at 0 dBm of the fiber input power, the dispersion tolerance in the method using the NRZ encoded signal is approximately twice as much as that of the method using the RZ encoded signal, while the dispersion tolerance in the method using the optical duobinary encoded signal is approximately four times as much as that of the method using the NRZ encoded signal. Here, the optimum dispersion is approximately 0 ps/nm.
However, when the intensity of light incident on the optical fiber transmission path is increased, the dispersion tolerance of the optical duobinary transmission method is degraded, and in particular, the total dispersion, which is optimum in a low-power region, is considerably degraded in the vicinity of 0 ps/nm. When the fiber input power exceeds 5 dBm, the amount or degree of degradation of the eye opening may exceed 1 dB.
On the other hand, in the methods using the NRZ and RZ encoded signals, the optimum dispersion is shifted towards the positive dispersion side according to the increase of the fiber input power, and the point of 0 ps/nm, which is the optimum point under the low power condition such as 0 dBm, is positioned near an end in the dispersion tolerance width (or margin), and the tolerance margin is considerably decreased if the incident optical power is further increased. This is because a frequency chirp is added to the optical signal due to the nonlinear optical effects in the optical fiber, Additionally, the dispersion tolerance itself is very small such as ¼ or ⅛ in comparison with the optical duobinary transmission method; thus, the system design itself is difficult and system optimization is also difficult when the system is introduced into practical use.
As explained above, in the transmission methods using the optical duobinary, NRZ, and RZ encoded signals, the optimum point of dispersion tolerance shifts with respect to a wide fiber input power range. This makes the system design complicated and disturbs the speedy introduction and stable operation of the system. That is, in the design of the optical transmission system, it is necessary to consider the optimum dispersion which varies depending on the fiber input power, and thus the design is complicated.
Additionally, when the optical transmission system is installed, the dispersion of the optical fiber transmission path is measured using a dispersion measurement device, and an optimum dispersion (generally, 0 ps/nm, but a slightly shifted value if the transmitted signal is chirped) is defined so as to establish the system. However, only the dispersion of the optical fiber can be acquired in the above measurement of dispersion; thus, it is difficult to follow the variation of the optimum dispersion specific to each transmission system which employs a specific encoded signal. In other words, in the conventional methods, the effective dynamic range of the incident light is small. Therefore, in the optical transmission system employing a conventional method, the bit rate or transmission distance must be limited.
Also as explained above, in each of the transmission methods using the optical duobinary, NRZ, and RZ encoded signals, the dispersion tolerance is considerably degraded according to an increase of the fiber input power. This prevents the stable operation of the optical transmission system.